A portable tool of this kind is disclosed in published German patent application Ser. No. DE-OS 32 22 457 wherein a plastic crankcase containing the crankshaft bearing means is attached to the cylinder by means of threaded fasteners which pass through the crankcase from below. With this configuration, a substantial portion of the reaction forces occurring in the region of the crankshaft are directed to the crankcase during the operation of the motor. In order to resist the stress occurring thereby, the crankcase must be produced from a high quality plastic material since the operational reliability of the tool is otherwise adversely affected.